The present invention relates to a method and apparatus for optically inspecting the surface of an article for defects. The invention is particularly useful for optically inspecting patterned semiconductor wafers used in producing integrated-circuit dies or chips, and the invention is therefore described below particularly with respect to this application.
The inspection of unpatterned semiconductor wafers for surface-lying particles is relatively simple and can be easily automated. In one known type of such system, the wafer is scanned by a laser beam, and a photodetector detects the presence of a particle by collecting the light scattered by the particle. However, the inspection of patterned semiconductor wafers for defects in the pattern is considerably more difficult because the light scattered by the pattern overwhelms the light scattered from the particles or defects, thereby producing high rates of false alarms.
The existing inspection systems for inspecting patterned wafers are generally based on analysing high resolution two-dimensional images of the patterned wafer utilizing an opto-electric converter, such as a CCD (charge-coupled device), on a pixel-by-pixel basis. However, the extremely large number of pixels involved makes such systems extremely slow. For this reason, the inspection of patterned wafers is done at the present time almost only for statistical sampling purposes. As a result, microdefects in patterned semiconductor wafers remain largely undetected until a considerable number of such wafers have been fabricated and have begun to exhibit problems caused by the defects. The late discovery of such defects can therefore result in considerable losses, low yields, and large downtimes.
There is therefore an urgent need to inspect patterned semiconductor wafers at relatively high speeds and with a relatively low false alarm rate in order to permit inspection during or immediately after the fabrication of the wafer so as to quickly identify any process producing defects and thereby to enable immediate corrective action to be taken. This need is made even more critical by the increasing element density, die size, and number of layers in the integrated circuits not being produced from these wafers, and now being designed for future production, which requires that the number of microdefects per wafer be drastically reduced to attain a reasonable die yield.